By day they are human, but in their spare time they become mythical heroes such as wizards, dwarfs and blood-elves. At the weekend, in California, 15,000 of them logged out of World of Warcraft, the world’s biggest online game, to gather in a hall the size of an aircraft hangar for the “big nerdfest” that is Blizzcon.

They were drawn to the Anaheim Convention Centre in California by their love of the virtual worlds they inhabit. Blizzcon is part video game carnival, part trade fair and part religious pilgrimage, organised by Blizzard Entertainment, which makes World of Warcraft and the real-time strategy game Starcraft.

Almost 11 million people pay £7 a month to play World of Warcraft and dedicated fans have flown in from 27 countries around the world to attend Blizzcon, where they can discuss the intricate details of Blizzard’s games with the game-makers, get the exclusive chance to play new games before general release, dress up as their favourite characters and compete against one another.

At the opening ceremony, in reality little more than a glorified press conference, the atmosphere is intense and the noise deafening. The screams reach a peak when Mike Morhaime, the Blizzard chief executive, takes the stage. He’s a gentle-looking man wearing a blazer — the crowd greets him like a rock star.

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“We are on the eve of a historic event next month, and I don’t mean the presidential election,” he says. The packed hall roars in agreement. For this mob, something far more important than electing the leader of the free world is taking place in November — “Northrend will be open.”

Northrend is a new continent in World of Warcraft and forms part of the latest expansion of the game, which goes on sale next month. The release of Wrath of the Lich King will be the biggest event in computer gaming this year. Fans will queue for hours to buy a copy. Gamers expect no less than what Blizzard aims to achieve: the greatest computer game ever created.

The first World of Warcraft game was released four years ago. Today, it is the world’s biggest “massively multi-player online game”, or MMO. It has all the elements of a sci-fi fantasy world you might expect; with dragons, gnomes, orcs and such. It is a knowing homage to the likes of Lord of the Rings and Dungeons and Dragons. Players spend hours going on quests and doing battle with one another in order to build up their character into something akin to a small god. In an ever-evolving virtual world that is illustrated in rich detail and can be explored endlessly.

Paul Sams, a senior Blizzard executive, said the key to the game was its accessibility. “It’s easy to learn but hard to master,” he added.

Walking around the convention floor, there is a certain Blizzcon chic that is noticeable. The crowds are filled with loads of chubby, young, and pale men. They wear black t-shirts, a back-to-front cap, and sport an ill-advised goatee. There are endless queues for everything from nachos to the gift store. Disneyland just down the road won’t see anything like it.

Blizzcon has been growing in popularity since it began in 2005. This year the frenzy to buy tickets caused the online booking system to crash. Once the problem was fixed, the $150 (£90) tickets sold out in 15 minutes.

The longest queues are for the chance to play new games. After hours of waiting, gamers finally get their chance to play at one of the hundreds of computers screens, lined up in banks. The whole event takes place in semi-darkness; natural light is not let in, ensuring that glare does not distract the players from the digital carnage.

Others quietly play fantasy card games with each other, or take part in intense Q&A sessions with the game creators. Some honest fans admit that they main reason they have come is to get hold of the much-wanted Blizzcon goodie-bag. What makes the bag so special? It includes a polar bear mount, an extremely rare character which players can use to ride upon, like a horse or a camel, in World of Warcraft. The mount is valuable, both in the game and in real life. It is already being sold for anything up to $300 on eBay.

On one stage, hundreds gather to watch professional Warcraft and Starcraft matches. Big screens show the action while commentators describe the battle excitedly. Players are locked away from the crowd in sound-proof booths. The action is furious. The contestants tap on their keyboards and click on their mouse about eight times a second. To the outsider, it all makes very little sense.

Meanwhile, there are thousands more watching the proceedings at home, having paid about $40 to view it on pay-per-view television. Critics describe World of Warcraft as the greatest video game creation, a masterpiece in art and engineering. But its genius is that it has evolved to become a deeply social and communal experience.

“Two years ago on Father’s Day, my son gave me World of Warcraft,” says Jim York, 61, from Los Angeles, who was at Blizzcon with his 30-something adult son, who lives around about 350 miles away from him in San Francisco. “Now about twice a month, we go online and go on quests together. It’s become a father-son experience.”

Blizzcon is home for this growing online community. Almost everyone here says they have come out of a sense of belonging. People who have become friends by playing together online meet for the first time at the convention.

This includes James Taplin, 29, from Dorset, England, who met his online friend Nathan, from Birmingham, just before flying to Blizzcon last week. “It’s all about the togetherness,” said Mr Taplin. “We talk to each other for a while about our day, then we put that to one side and go kill some monsters."

In a world of their own

— World of Warcraft’s 11 million monthly subscribers make it the biggest multiplayer online game in the world, with 62 per cent of the market

— In 2005 the world contracted a virtual plague that infected millions of characters and drew international attention because of its resemblence to real-life epidemics

— A baby reportedly suffocated in South Korea while her parents played World of Warcraft in a local café

— The Chinese Government restricts under18s to three hours playing time, after which the player is expelled for the game

— American psychologists have estimated that up to 40 per cent of World of Warcraft players are addicted to it

— World of Warcraft has appeared in South Park and The Simpsons cartoons, as well as inspiring two board games, a comic book and a music album

— World of Warcraft is advertised on television by William Shatner, Mr T and Jean Claude Van Damme